1. Field of the Invention
The present invention relates to an apparatus and a method for detecting a decrease in a tire air pressure, and a program for detecting a decrease in a tire air pressure.
2. Description of the Related Art
Run flat tires can be used to enable, even when the tires are punctured and air therein is lost, a running for a distance of about 100 km. Thus, by the run flat tires, it is possible to avoid a situation where the vehicle is stopped at a dangerous place such as a road having a lot of traffic to exchange a tire or to fix the puncture. Thus, the run flat tires allow the user to drive the vehicle to the nearest service station to have the maintenance of the vehicle safely. Furthermore, the run flat tires also eliminate the need to store a spare tire in the vehicle, thus providing advantages such as an increased trunk space and improved fuel consumption by the vehicle having a lighter weight.
On the other hand, in the case of a tire like the run flat tire by which the vehicle can run even when the air pressure of the tire is zero, a conventional method focusing attention on a change in the tire rotation radius (see Japanese Unexamined Patent Publication No. 305011/1988 for example) has a difficulty in judging a decline in the air pressure. Thus, when the vehicle continuously runs at a high speed with a declined air pressure, the tires may burst.
Thus, in order to detect an abnormal air pressure of a tire when the air pressure of the tire is completely zero, a method has been suggested according to which the variations in the accelerations of the respective tires are compared (see Japanese Unexamined Patent Publication No. 2007-182211 for example).
In this method according to Japanese Unexamined Patent Publication No. 2007-182211, the variations in the acceleration of the respective tires are calculated and are compared. When the comparison result shows that a certain tire has an acceleration variation greater than those of the other tires, it is judged that the tire has an abnormal air pressure.
In the case of the method according to Japanese Unexamined Patent Publication No. 2007-182211 however, a disadvantage is caused as described below. Specifically, when a snow chain is attached to one tire among four wheels having a normal pressure, the acceleration variation calculated based on the rotation velocity of the tire with the snow chain differs from the acceleration variations calculated based on the rotation velocities of the other tires. Thus, in spite of the normal internal pressure, a wrong judgment is made that the tire with the snow chain attached thereto has an abnormal air pressure.
The present inventor has keenly researched a method to accurately detect, without issuing a false alarm as described above, that the air pressure of one wheel tire is completely zero. Through the research, the present inventor has found that, when the air pressure of one wheel tire is completely zero, there are two cases, namely a case where the tire has a rotation velocity faster than those of other tires and a case where the tire has a rotation velocity slower than those of other tires on the contrary. The present inventor has also found that the degree at which the rotation velocity becomes slower is more remarkable than in the case where a snow chain is attached to one wheel tire, thus reaching the present invention.